Phytases (myo-inositol hexakisphosphate phosphohydrolases; EC 3.1.3.8) are enzymes that hydrolyze phytate (myo-inositol hexakisphosphate) to myo-inositol and inorganic phosphate and are known to be valuable feed additives.
A phytase was first described in rice bran in 1907 [Suzuki et al., Bull. Coll. Agr. Tokio Imp. Univ. 7, 495 (1907)] and phytases from Aspergillus species in 1911 [Dox and Golden, J. Biol. Chem. 10, 183-186 (1911)]. Phytases have also been found in wheat bran, plant seeds, animal intestines and in microorganisms [Howsen and Davis, Enzyme Microb. Technol. 5, 377-382 (1983), Lambrechts et al., Biotech. Lett. 14, 61-66 (1992), Shieh and Ware, Appl. Microbiol. 16, 1348-1351 (1968)].
The cloning and expression of the phytase from Aspergillus niger (ficuum) has been described by Van Hartingsveldt et al., in Gene, 127, 87-94 (1993) and in European Patent Application, Publication No. (EP) 420 358 and from Aspergillus niger var. awamori by Piddington et al., in Gene 133, 55-62 (1993).
Cloning, expression and purification of phytases with improved properties have been disclosed in EP 684 313. However, since there is a still ongoing need for further improved phytases, especially with respect to the activity properties, it is an object of the present invention to provide such improvements.
Accordingly, this invention is directed to a process for the production of a modified phytase with a desired property improved over the property of the corresponding unmodified phytase which comprises:
(a) determining the three dimensional structure of the unmodified phytase and of a second phytase which has the desired property by aligning the amino acid sequences of said phytases with the amino acid sequence of a third phytase which is the phytase of Aspergillus niger and using the three dimensional structure of the phytase of Aspergillus niger as a template based on the alignment to determine said three dimensional structures;
(b) determining from the structures of step (a) the amino acids of the active sites of the unmodified phytase and of the second phytase having the desired property which active site provides the desired property and comparing the amino acids which form the active sites to identify which amino acids are different in the active site of the second phytase from the amino acids in the active site of the unmodified phytase;
(c) constructing a DNA sequence coding for the modified phytase by obtaining the DNA sequence of the unmodified phytase and changing the nucleotides coding for the active site which provides the desired property for said unmodified phytase so that at least one of the amino acids in the active site which provides the desired property is substituted by one of the amino acids which was identified as being different in step (b);
(d) integrating such a DNA sequence into a vector capable of expression in a suitable host cell; and
(e) transforming the suitable host cell by the DNA sequence of step (c) or the vector of step (d), growing said host cell under suitable growth conditions and isolating the modified phytase from the host cell or the culture medium.
Either or both of the unmodified phytase and the phytase with the desired property may be of eukaryotic origin, especially of fungal origin. Such phytases are preferably of Aspergillus origin, for example phytase from Aspergillus fumigatus. In a preferred process, the phytase with the desired property is a phytase from Aspergillus terreus. In another preferred process, the unmodified phytase is a phytase of Aspergillus fumigatus and the phytase with the desired property is the Aspergillus niger phytase. In yet another preferred process, the unmodified phytase is a phytase of Aspergillus fumigatus and the phytase with the desired property is the Aspergillus terreus phytase.
Also part of this invention is a modified phytase with a specific activity improved over the specific activity of the corresponding unmodified phytase (for example Aspergillus fumigatus) wherein the amino acid sequence of the corresponding unmodified phytase has been changed by one or more of deletion, substitution and addition by one or more amino acids to obtain the amino acid sequence of the modified phytase. A preferred phytase has an amino acid sequence homologous to that of the phytase of Aspergillus niger (SEQ ID NO:1) and has an amino acid sequence that has been changed in at least one amino acid position selected from the following amino acid positions which correspond to positions of the amino acid sequence of the phytase of Aspergillus niger: 27, 66, 71, 103, 140, 141, 188, 205, 234, 238, 274, 277, 282, 340 and 424, in particular wherein the amino acid position is selected from 27, 66, 140, 205, 274, 277, 282, and 340.
A preferred modified phytase has an amino acid sequence which has been changed at position 27 alone or in addition to other of the above positions, in particular at least at position 66 and/or position 140. Thus preferred phytases are modified at position 27 and 66 or 27 and 140.
For any such phytase, the amino acid at position 27 may be replaced by a specific amino acid selected from one of the following groups:
a) Ala, Val, Leu, Ile; or b)Thr; or c) Asn.
Particular modified phytases of this invention are characterized by at least one of the following changes in amino acids at positions: Q27L, Q27N, Q27T, Q271, Q27V, Q27A, Q27G, S66D, S140Y, D141G, A205E, Q274L, G277D, G277K, Y282H and/or N340S.
Also part of this invention are polynucleotides comprising a DNA sequence coding for the modified phytases produced by the above method. Polynucleotides comprising DNA sequences coding for the phytases described above which are modified at particular amino acid positions are included.
Also included are vectors, especially expression vectors, which contain the polynucleotides of this invention, and host cells which contain these polynucleotides directly or within a vector.
Another aspect of this invention is a food or feed composition which contains modified phytases described above.